610 UNGULATES. 



ridges with an anterior and posterior basal ridge ; its contour is 

 almost square. The third milk molar has a greater antero- 

 posterior extent, and supports three transverse eminences with 

 an anterior and posterior basal ridge, the anterior ridge being 

 developed into a pointed tubercle at its outer end. The crowns 

 of the first two permanent molars, or the premolars, were dis- 

 covered by Dr. Kaup in their closed alveoli above the second 

 and third deciduous molars, in a portion of the upper jaw.(l) 

 These two premolars were in place, fully developed, with the crowns 

 a little abraded, in the entire cranium subsequently discovered. (2) 

 They conform to the general rule in being more simple than the 

 teeth which they displace and succeed. The first upper premolar 

 supports a longitudinal ridge on the outer side of the crown, and 

 two mamilloid tubercles with confluent bases along the inner side 

 of the crown, which is surrounded, except at its outer part, by a 

 basal ridge. The unworn summits of both the ridge and tubercles 

 are divided into smaller tubercles by a series of notches. The 

 crown of the second premolar supports four tubercles, the outer 

 ridge being deeply cleft, and the two anterior tubercles are united 

 by a continuous ridge, which converts them into a transverse 

 eminence, like those which characterise the true molar teeth. 

 The transverse diameter of the second premolar exceeds the 

 antero-posterior one, the proportions being the reverse of those of 

 the deciduous molar which it displaces. The first true molar 

 repeats the structure of the hindmost deciduous molar, its crown 

 having a disproportionate antero-posterior extent and supporting 

 three transverse eminences, with an anterior, posterior, and internal 

 basal ridge. The Dinothere resumes the Tapiroid character and 

 differs essentially from the Mastodon inasmuch as the posterior 

 molars, instead of having an increased antero-posterior extent and 

 more complex crowns, increase only in thickness and support two 

 instead of three transverse eminences : they have also an anterior 

 and a posterior basal ridge. 



In the lower as in the upper jaw it is uncertain whether the 



(1) Akten der Urwelt, 8vo. 1841, tab. viii, fig. 1. 



(2) Ibid. tab. vii. 



